Boreholes are formed in subterranean formations for various purposes including, for example, extraction of oil and gas from subterranean formations and extraction of geothermal heat from subterranean formations. Boreholes may be formed in subterranean formations using earth-boring tools such as, for example, drill bits.
To drill a borehole with a drill bit, the drill bit is rotated and advanced into the subterranean formation under an applied axial force, commonly known as “weight on bit,” or WOB. As the drill bit rotates, the cutters or abrasive structures thereof cut, crush, shear, and/or abrade away the formation material to form the borehole, depending on the type of bit and the formation to be drilled. A diameter of the borehole drilled by the drill bit may be defined by the cutting structures disposed at the largest outer diameter of the drill bit.
The drill bit is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which comprises a series of elongated tubular segments connected end-to-end that extends into the borehole from the surface of the formation. Often various subs and other components, such as a downhole motor, a steering sub or other assembly, a measuring while drilling (MWD) assembly, one or more stabilizers, or a combination of some or all of the foregoing, as well as the drill bit, may be coupled together at the distal end of the drill string at the bottom of the borehole being drilled. This assembly of components is referred to in the art as a “bottom hole assembly” (BHA).
The drill bit may be rotated within the borehole by rotating the drill string from the surface of the formation, or the drill bit may be rotated by coupling the drill bit to a down-hole motor, which is also coupled to the drill string and disposed proximate to the bottom of the borehole. The downhole motor may comprise, for example, a hydraulic Moineau-type motor having a shaft, to which the drill bit is mounted, that may be caused to rotate by pumping fluid (e.g., drilling fluid or “mud”) from the surface of the formation down through the center of the drill string, through the hydraulic motor, out from nozzles in the drill bit, and back up to the surface of the formation through an annulus between the outer surface of the drill string and the exposed surface of the formation within the borehole. As noted above, when a borehole is being drilled in a formation, axial force or “weight” is applied to the drill bit (and reamer device, if used) to cause the drill bit to advance into the formation as the drill bit drills the borehole therein.
It is known in the art to employ what are referred to as “depth-of-cut control” (DOCC) features on earth-boring drill bits which are configured as fixed-cutter, or so-called “drag” bits, wherein polycrystalline diamond compact (PDC) cutting elements, or cutters, are used to shear formation material. For example, U.S. Pat. No. 6,298,930 to Sinor et al., issued Oct. 9, 2001, discloses rotary drag bits that including exterior features to control the depth of cut by PDC cutters mounted thereon, so as to control the volume of formation material cut per bit rotation as well as the reactive torque experienced by the bit and an associated bottom-hole assembly. The exterior features may provide sufficient bearing area so as to support the drill bit against the bottom of the borehole under weight-on-bit without exceeding the compressive strength of the formation rock. However, such depth-of-cut control features may not be well suited for drilling all borehole segments during directional drilling applications. For example, when drilling in slide mode (i.e., on-center drilling and directional drilling) to form a non-linear borehole segment, it may be desirable to maintain a relatively small depth of cut to improve steerability; however, conventional depth-of-cut control features may hinder efficient drilling in rotate mode (i.e., off-center drilling and vertical drilling) wherein a higher rate of penetration (ROP) is desirable.
In view of the foregoing, improved drill bits for directional drilling applications, improved methods of manufacturing such bits and improved methods of directional and off-center drilling applications would be desirable.